There are a variety of devices used to treat fractures of the femur, humerus, tibia, and other long bones. For example, fractures of the femoral neck, head, and intertrochanteric region have been successfully treated with a variety of compression screw assemblies, which include generally a compression plate having a barrel member, a lag screw and a compressing screw. Examples include the AMBI® and CLASSIC™ compression hip screw systems offered by Smith & Nephew, Inc. In such systems, the compression plate is secured to the exterior of the femur, and the barrel member is inserted in a predrilled hole in the direction of the femoral head. The lag screw has a threaded end, or another mechanism for engaging bone, and a smooth portion. The lag screw is inserted through the barrel member so that it extends across the break and into the femoral head. The threaded portion engages the femoral head. The compression screw connects the lag screw to the plate By adjusting the tension of the compression screw, the compression (reduction) of the fracture can be varied. The smooth portion of the lag screw is free to slide through the barrel member to permit the adjustment of the compression screw. Some assemblies of the prior art use multiple screws to prevent rotation of the lag screw relative to the compression plate and barrel member and also to prevent rotation of the femoral head on the lag screw.
Intramedullary nails in combination with lag screws or other screw assemblies have been successfully used to treat fractures of the femur, humerus, tibia, and other long bones as well. A significant application of such devices has been the treatment of femoral fractures. One such nailing system is the IMHS® system offered by Smith & Nephew, Inc., and covered at least in part by U.S. Pat. No. 5,032,125 and various related international patents. Other seminal patents in the field include U.S. Pat. Nos. 4,827,917, 5,167,663, 5,312,406, and 5,562,666, which are all assigned to Smith & Nephew, Inc. These patents are all hereby incorporated by reference A typical prior art intramedullary nail may have one or more transverse apertures through its distal end to allow distal bone screws or pins to be screwed or otherwise inserted through the femur at the distal end of the intramedullary nail. This is called “locking” and secures the distal end of the intramedullary nail to the femur. In addition, a typical intramedullary nail may have one or more apertures through its proximal end to allow a lag screw assembly to be screwed or otherwise inserted through the proximal end of the intramedullary nail and into the femur. The lag screw is positioned across the break in the femur and an end portion of the lag screw engages the femoral head. An intramedullary nail can also be used to treat shaft fractures of the femur or other long bones.
As with compression hip screw systems, intramedullary nail systems are sometimes designed to allow compression screws and/or lag screws to slide through the nail and thus permit contact between or among the bone fragments. Contact resulting from sliding compression facilitates faster healing in some circumstances. In some systems, two separate screws (or one screw and a separate pin) are used in order, among other things, to prevent rotation of the femoral head relative to the remainder of the femur, to prevent penetration of a single screw beyond the femoral head, and to prevent a single screw from tearing through the femoral neck and head. When an additional screw or pin is used, however, unequal forces applied to the separated screws or pins can cause the separate screws or pins to be pressed against the sides of the holes through which the separate screws or pins are intended to slide. This may result in binding, which reduces the sliding of the screws or pins through the nail. Conversely, a problem can result from excessive compression of the femoral head toward or into the fracture site. In extreme cases, excessive sliding compression may cause the femoral head to be compressed all the way into the trochanteric region of the femur.
Furthermore, overly rigid nails sometimes generate periprosthetic fractures in regions away from a fracture site. Therefore, it is important that intramedullary nails be adequately flexible in comparison to the bones in which they are implanted.
The harder, generally outer portion of a typical bone is referred to as cortical bone. Cortical bone is usually a structurally sound load-bearing material for support of an implant. A cross-section of a long bone that shows the typical anatomical shape of cortical bone generally reveals a non-circular ring of cortical bone which surrounds a medullary canal. Accordingly, the medullary canal generally features a non-circular cross section. Intramedullary nails of the prior art, however, are usually round or square in cross-section, and therefore not anatomically consistent with the cortical bone or the medullary canal. Some have addressed this problem by reaming the medullary canal of the bone with a round reamer in order to cause the nail to fit the cortical bone. This approach, however, can remove significant portions of healthy cortical bone.
The problem of providing an effective load bearing physical relationship between an implant and cortical bone in the proximal femur has been addressed in the art of hip replacement devices. Various hip stems have been developed which feature generally non-circular cross sections along their length, in order better to fit the anatomically shaped cortical bone of the proximal femur and thus more evenly and effectively distribute the load between the stem and the bone. However, none of these hip stems have been incorporated into a nail or configured to accept a screw or screws useful in repairing substantially all of the portions of the treated bone. Instead, hip stems as a general matter have been considered as a device for replacing portions of a long bone, and designed and used for that purpose. For example, the typical application of a hip stem includes completely removing a femoral head and neck, implanting a hip stem, and using the hip stem to support an artificial femoral head.
In summary, and without limitation, the foregoing shows some of the shortcomings of the state of the art in this field. Among other things, what is needed is an orthopaedic implant system that includes a superior sliding screw or other mechanism for applying compression across a fracture. Some embodiments would also provide a sliding screw or other mechanism that obtains adequate bone purchase while reducing the incidence of cut-out, rotational instability, and excessive sliding. An anatomically appropriately shaped implant for achieving improved cortical bone contact would also be advantageous. Where the implant is an intramedullary nail, the nail would provide for reduced reaming and removal of healthy bone. An improved nail may also have a cross-section that provides a greater area of material on the side of the nail that is placed under a greater tensile load when the nail is subjected to a typical bending load. Additionally, an improved implant system could include a sliding screw in combination with intramedullary nails of various designs, or in combination with plates. Combinations of any of these with each other or combinations of each other, and/or with other devices or combinations of them also present opportunities for advancement beyond the state of the art according to certain aspects of the present invention.